U.S. Pat. No. 5,323,404 (incorporated herein by reference) discloses cascaded optical fiber Raman lasers and amplifiers (collectively "devices"). The devices comprise one or more pairs of reflectors, spaced apart, with reflectors of each pair having the same center wavelength. The reflectors typically are in-line refractive index Bragg reflectors "written" in SiO.sub.2 -based optical fiber with Ge-containing core, each pair of reflectors providing an optical cavity in the fiber. Cavities that are neighbors in wavelength differ in wavelength by one Stokes shift, the longer wavelength lying within the Raman gain spectrum. See "Optical Fiber Communications", S. E. Miller et al., editors, Academic Press 1979, p. 127 for the Raman gain spectrum of silica. Radiation that is resonating in one of the optical cavities will be referred to as a "Stokes line". See also U.S. patent applications Ser. No. 08/897,195, filed Jul. 21, 1997 by DiGiovanni et al., and Ser. No. 08/871,023, filed Jun. 6, 1997 by W. A. Reed et al., both incorporated herein by reference.
The optical fiber Raman devices comprise means for coupling pump radiation of wavelength .lambda..sub.p into the optical fiber, and are adapted for emission of radiation of a wavelength longer than .lambda..sub.p. For further detail on the prior art devices, see the '404 patent and the '195 patent application.
In many potentially important applications of cascaded optical fiber Raman devices, the number of reflector pairs exceeds two. For instance, a laser having 1.5 .mu.m output and utilizing 1.1 .mu.m pump radiation typically requires 5 reflector pairs. Not only is a device with so many reflectors difficult to manufacture since the members of each pair of reflectors have to have essentially the same center wavelength, but the unwanted attenuation due to the multiplicity of reflectors can be significant. For instance, a Stokes line can experience up to 0.5 dB loss as the radiation passes through the reflectors for the other wavelengths.
Cascaded Raman devices are potentially desirable for high power applications, e.g., lasers as pump sources for rare earth doped fiber amplifiers, or for free space communications, and amplifiers as amplifiers in optical communications systems. Thus, it would be highly desirable to have available devices having higher output power and/or lower threshold than prior art cascaded Raman devices. This application discloses such devices.
As is known to those skilled in the art, the Stokes shifts are determined by the Raman gain spectrum of the fiber material and by frequency-selective elements, e.g., fiber Bragg gratings, that determine the amount of wavelength shift.